In recent years, in radio communications, especially in mobile communications, in addition to voice, various information including image is transmitted. Along with this diversification of media of transmission, technologies for enabling high-speed transmission of information are in demand. The OFDM (Orthogonal Frequency Division Multiplexing) scheme is a multicarrier transmission scheme having features of high spectrum efficiency and low inter-symbol interference in the multipath environment, and is currently a focus of attention as a transmission scheme that may meet the above-noted demand.
In a radio communication system adopting the OFDM scheme, for example, at the transmitting side, a plurality of the same data symbols are generated by duplicating a data symbol (hereinafter this process will be referred to as “repetition”) in the frequency domain (that is, on a plurality of subcarriers), and, at the receiving side, these symbols are combined (using, for example, maximum ratio combining). In the following description, OFDM schemes involving the above processing will be referred to as “repetition OFDM”. In this case, the received quality of data symbols (for example, SNR: Signal to Noise Ratio, and SIR: Signal to Interference Ratio) is improved by combining. A subcarrier may be referred to as a “tone”.
Also, to implement high-speed transmission, it is important to maintain the reception performance at the receiving side at a certain level or higher, and, for this purpose, adequate control for accurate transmission channel estimation is necessary. For example, patent document 1 proposes a method for controlling transmission power for pilot symbols used in transmission channel estimation and controlling the accuracy transmission channel estimation. Specifically, patent document 1 proposes controlling transmission power for pilot symbols according to the modulation scheme and coding rate of data symbols. For example, a modulation scheme that is less robust to frequency selective fading requires high accuracy in transmission channel estimation, and, consequently, the transmission power of pilot symbols is increased. On the other hand, a modulation scheme that is more robust to frequency selective fading does not require so much accuracy in transmission channel estimation, and, consequently, the transmission power of pilot symbols is decreased.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-60609